Diffractive optical element for providing favorable multi-mode fiber launch and reflection management

ABSTRACT

A light transmission system includes a laser, an optical fiber, and a transfer lens. The transfer lens transfers light emitted by the laser into the optical fiber. The transfer lens includes a diffractive surface for receiving and collimating the light originating form the laser. The diffractive surface is defined by a surface function that includes a first phase function having angular symmetry and a second phase function having radial symmetry. The second phase function includes a cusp region with a discontinuous slope therein. The transfer lens provides reflection management so that light reflected from the end of the optical fiber is not focused at a location at which light is emitted by the laser and also favorable launch conditions so that light launched into the optical fiber avoids index anomalies along the axis of the optical fiber.

FIELD OF THE INVENTION

The present invention relates generally to optics, and moreparticularly, to a diffractive optical element that provides favorablemulti-mode fiber launch and reflection management.

BACKGROUND OF THE INVENTION

A vertical cavity surface emitting laser (VCSEL) emits light in a beamvertically from it surface. Light emitted from a VCSEL is typicallyfocused by a transfer lens into an optical fiber and used for thetransmission of data. Transmission technology such as Gigabit Ethernettechnology utilizes VCSELs and multimode fiber optic cabling.

The ever-increasing data rate across multimode fiber optic systemsrequires more sophisticated coupling optics for the transmitter moduleto satisfy the required bit-error rate.

There are two important considerations in the design of a transferlens: 1) reflection management, and 2) creation of a favorable launchcondition. The first design consideration of reflection management seeksto minimize the amount of light that is reflected back from the surfaceof the optical fiber (referred to as “back reflections” or feedback) anddirected to the light source (e.g., the laser). When reflections are notmanaged properly, the back reflections can cause stability problems forthe laser source. Specifically, if these back reflections are notcontrolled or reduced, the laser can become de-stabilized and mayoperate with a noisy output signal. For example, when too much power iscoupled back into the laser from the reflection from the end of theoptical fiber, instabilities occur in the laser, and the output poweroscillates up and down, thereby causing extra and damaging amounts ofjitter as the received signal pulses. In other words, instability in thelaser causes erroneous data signals.

Furthermore, the increased noise in the laser that is induced by thecoupling lens can lead to a power penalty in the optical budget of thedata link as high as 2.5 dB. It is evident that the increased powerpenalty due to the back reflections represents a significant fraction ofthe total link power budget which, for a 2.5 Gbit/sect data rate, is onthe order of about 8 dB. This adverse effect of back reflections orfeedback becomes more pronounced and significant for higher data ratesystems. For example, the power budget for a 10 Gbit/sec link becomeseven more taxed than the 2.5 Gbit/sec link.

Second, it is important that the transfer lens design provide afavorable launch condition at the fiber interface in order to maximizethe bandwidth-distance product of the system. For example, for astandard 50 micron graded-index fiber, a 2.5 Gbit/sec link requires abandwidth-distance product of 500 MHz*km. Similarly, for a 10 Gbit/seclink, the fiber needs to support a product of 2.2 Ghz*km.

A favorable launch condition should increase bandwidth of the system andis robust to lateral offsets (i.e., misalignment between the laser andthe fiber). One approach to improve favorable launch condition is toavoid launching the light along the very center of the fiber. A reasonfor avoiding the center of the fiber is that many fibers have defectsalong the center of the fiber due to manufacturing limitations.Furthermore, tolerance for lateral offsets is desirable to compensatefor any misalignment between the laser and the fiber. Otherwise,misalignment in the system (e.g., misalignment between the optical fiberand transfer lens or the misalignment between the transfer lens and thelaser) may cause the light from the laser to miss the optical fiber.

Unfortunately, the prior art transfer lens designs have shortcomings ineither addressing back reflections or providing favorable launchconditions. These shortcomings and disadvantages stem primarily fromconstraints and difficulties in lens fabrication.

Diffractive Vortex Lens for Mode-Matching Graded Index Fiber

There have been some attempts to use diffractive elements as couplingoptics to launch light into graded index fiber. One such study isreported by E. G. Johnson, J. Stack, C. Koehler, and T. Suleski in theDiffractive Optics and Micro-Optics, Optical Society of America (OSA)Technical Digest, pp. 205-207, Washington, DC, 2000, in an articleentitled, “Diffractive Vortex Lens for Mode-Matching Graded IndexFiber.” This publication describes an approach that utilizes adiffractive element to match the phase of the launched light intospecific modes of the graded index fiber.

While these prior art approaches provide tolerable results for idealpoint light sources (i.e., light that has a simple distribution and isperfectly coherent), these approaches do not adequately addressapplications that employ light sources with more complex lightdistributions (e.g., a multi-mode laser). In these specific real-worldapplications, the prior transfer lens suffer from more destabilizingfeedback due to poor management of back reflections, unfavorable launchconditions stemming from larger amounts of on-axis energy, or both.

Based on the foregoing, there remains a need for a transfer lens thatsimultaneously reduces back reflection and provides favorable launchconditions and that overcomes the disadvantages set forth previously.

SUMMARY OF THE INVENTION

According to one embodiment of the present invention, a transfer lensfor transferring light emitted by a light source (e.g., a laser) into alight conducting medium (e.g., an optical fiber) is provided. Thetransfer lens includes a diffractive surface for receiving andcollimating the light originating from the light source. The diffractivesurface is defined by a surface function that includes a first phasefunction having angular symmetry and a second phase function havingradial symmetry. The second phase function includes a cusp region with adiscontinuous slope therein. The transfer lens provides reflectionmanagement so that light reflected from the end of the optical fiber isnot focused at a location at which light is emitted by the light source.Furthermore, the transfer lens also provides favorable launch conditionsso that light launched into the optical fiber avoids index anomalies onthe axis of the optical fiber and at the core-cladding interface.

A further advantage of the transfer lens design of the present inventionis that the diffractive surface of the transfer lens provides reflectionmanagement and favorable launch conditions, which is particularlyadvantageous for multi-mode fiber optic systems.

In accordance with one embodiment of the present invention, a lighttransmission system that includes a laser, an optical fiber, and atransfer lens is provided. The transfer lens transfers light emitted bythe laser into the optical fiber. The transfer lens includes adiffractive surface for receiving and collimating the light originatingform the laser. The diffractive surface is defined by surface functionthat includes a first phase function having angular symmetry and asecond phase function having radial symmetry. The second phase functionincludes a cusp region with a discontinuous slope therein. The transferlens provides reflection management so that light reflected from the endof the optical fiber is not focused at a location at which light isemitted by the laser and also favorable launch conditions so that lightlaunched into the optical fiber avoids index anomalies along the axis ofthe optical fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements.

FIG. 1 is a simplified block diagram of an exemplary light transmissionsystem in which the transfer lens can be utilized in accordance with apreferred embodiment of the present invention.

FIG. 2A illustrates an exemplary first phase function having angularsymmetry. FIG. 2B illustrates an exemplary second phase function havingradial symmetry. FIG. 2C illustrates a function that combines the firstphase function of FIG. 2A and the second phase function of FIG. 2B inaccordance with a preferred embodiment of the present invention.

FIG. 3 illustrates a perspective view of a preferred diffractive surfaceof the transfer lens that has stair case features.

FIG. 4 illustrate cross-sections of exemplary phase functions havingradial symmetry and a cusp region that can be incorporated into thediffractive surface of the transfer lens in accordance with embodimentsof the present invention.

FIG. 5 is a spot diagram for the transfer lens of the present inventionfor illustrating favorable launch conditioning.

FIG. 6 is a spot diagram for the transfer lens of the present inventionfor illustrating reflection management.

FIG. 7 is a spot diagram for a prior art transfer lens for illustratingpoorer launch conditioning.

FIG. 8 is a spot diagram for a prior art transfer lens for illustratingpoor reflection management.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A diffractive optical element optimized for multi-mode fiber launch andfeedback control is described. In the following description, for thepurposes of explanation, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Itwill be apparent, however, to one skilled in the art that the presentinvention may be practiced without these specific details. In otherinstances, well-known structures and devices are shown in block diagramform in order to avoid unnecessarily obscuring the present invention.

Light transmission system 100

FIG. 1 is a simplified block diagram of an exemplary light transmissionsystem 100 in which the transfer lens 140 can be utilized in accordancewith a preferred embodiment of the present invention. The system 100includes a light source 110 (e.g., a laser) for emitting light, a lightconducting medium 120 (e.g., a fiber optic cable), and a transfer lens140 of the present invention for transferring light emitted by the lightsource 110 to the light conducting medium 120.

The light source 110 can be a semiconductor laser, such as a verticalcavity surface emitting laser (VCSEL), whose construction and operationare known to those of ordinary skill in the art. The light conductingmedium 120 can, for example, be a 50 micron multi-mode fiber or a 62.5micron multi-mode fiber, which are well-known types of fiber opticscables. The transfer lens 140 of the present invention further providesfavorable launch conditions and reflection management (also referred toherein as feedback management).

In the preferred embodiment, the transfer lens 140 includes adiffractive surface 150 for receiving and collimating the lightoriginating form the light source and also for providing feedbackmanagement and favorable launch conditions. The transfer lens 140includes an optical surface 160 (e.g., a refractive surface or adiffractive surface) for providing magnification of the light andfocusing the light onto the light conducting medium 120. The transferlens 140 of the present invention is described in greater detailhereinafter with reference to FIGS. 2-6.

One aspect of the transfer lens of the preferred embodiment is placingthe diffractive surface 150 inside the packaging so that the diffractivesurface 150 is less susceptible to dust and damage.

Diffractive surface 150

The diffractive surface 150 of the transfer lens shown in FIG. 1 isdefined by a surface function. In accordance with a preferred embodimentof the present invention, the surface function for the diffractivesurface 150 includes 1) a first phase function where the phase isdependent only on the polar angle coordinate of the aperture (hereinreferred to as a first phase function having “angular symmetry”), and 2)a second phase function where the phase is dependent only on the polarradial coordinate of the aperture (herein referred to as a phasefunction having “radial symmetry”) and further having a cusp region.Preferably, the second phase function is symmetric with respect to animaginary line drawn through the center of the aperture, and the cuspregion has a discontinuous slope contained therein.

Examples of a first phase function having angular symmetry and a secondphase function having radial symmetry are now described. FIG. 2Aillustrates an exemplary first phase function having angular symmetry(e.g., a spiral phase function where m_(S)=+3). m_(S) is a real number(from −INF to +INF) that describes how fast the phase changes as onetraverses a circle about the center of the aperture.

FIG. 2B illustrates an exemplary second phase function having radialsymmetry (e.g., a cone phase function where m_(C)=−2). m_(C) is a realnumber (from −INF to +INF) that describes how fast the phase changes asone traverses a radial line from the center of the aperture. The slopeof the cone controls the change in phase values from 0 at the center ofthe aperture to 2*pi*m at the edge. Further examples of second phasefunction having radial symmetry and a cusp region are described withreference to FIG. 4.

FIG. 2C illustrates a function that combines the first phase function ofFIG. 2A and the second phase function of FIG. 2B in accordance with apreferred embodiment of the present invention. In this embodiment, theparticular “m” values are selected as follows: m=+3 for phase functionhaving angular symmetry, and m=−2 for the phase function having radialsymmetry. The “m” values can be determined by balancing the couplingefficiency, misalignment tolerances and the amount of feedback. It isnoted that these “m” values may be adjusted to suit the requirements ofa particular optical application.

It is noted that the surface function for the surface 150 can includeother phase terms (e.g., a third phase function, a fourth phasefunction, etc.) to suit the requirements of a particular application.These additional phase functions or phase terms can include, forexample, lens functions, aberration control functions, prism functions,and grating functions. These terms are known to those of ordinary skillin the art and will not be described further herein.

In an alternative embodiment, the surface 150 can be a collimatingsurface for receiving and collimating the light originating and thediffractive aspect of the present invention can be incorporated insurface 160. In this embodiment, surface 160 is a diffractive surfacefor providing feedback management and favorable launch conditions forthe transfer lens.

FIG. 3 illustrates a perspective view of a preferred diffractive surfaceof the transfer lens that has stair case features. It is noted that thestair-like or step-like feature of the diffractive surface is especiallysuited for manufacture by standard semiconductor processes. For example,well-known lithography involving masks and etch processing steps can beemployed to realize the diffractive surface of transfer lens of thepresent invention. Alternatively, the diffractive surface can includecontinuous or smooth surface transitions. It is noted that thecontinuous or smooth surface transitions may lead to increasedperformance by the transfer lens of the present invention. However, thisalternative embodiment may require more complex processing steps thatinclude turning or milling the diffractive surface to achieve acontinuous slope.

FIG. 4 illustrates sectional views of exemplary phase functions havingradial symmetry and a cusp region that can be incorporated into thesurface function of the diffractive surface of the transfer lens inaccordance with embodiments of the present invention. A first phasefunction 410 has a cross section that features a generally concaveprofile. A second phase function 420 features a generally triangularcross-section. A first phase function 430 has a cross section thatfeatures a generally convex profile. It is noted that each of thesephase functions can be inverted, and the inverted phase function can beutilized to achieve a similar result. For example, a fourth phasefunction 440 is an inverted version of the first phase function 410

Preferred Embodiment

Preferably, the design of the diffractive element of the presentinvention includes at least two phase functions (i.e., the first phasefunction having angular symmetry combined with the second phase functionhaving radial symmetry) described above.

Describe the phase, φ, of all points within the lens aperture with polarcoordinates: ρ, θ, where σ is the distance of the point from the centerof the aperture, and θ is the angle of the point from the x-axis.

In the preferred embodiment, the surface function of the diffractiveelement of the present invention includes at least a first phasefunction having angular symmetry (e.g., a spiral phase function)combined with a second phase function having radial symmetry (e.g., acone phase function). For example, the surface function can be expressedas follows:

φ=m _(S)*θ+2πm _(C)*ρ.

The spiral phase function can be expressed as follows:

φ=m _(S)*θ

wherein ‘m_(S)’ is a real number, −INF to +INF, that describes how fastthe phase changes as one traverses a circle about the center of theaperture.

The cone phase function can be expressed as follows:

φ=2πm _(C)*ρ

where ‘m_(C)’ is a real number, −INF to +INF, that describes how fastthe phase changes as one traverses a radial line from the center of theaperture. ρ is a normalized radial coordinate so that ρ is equal to 1 atthe edge of the aperture, and ρ is equal to zero at the center of theaperture.

As noted previously, other phase functions or phase terms may be addedto the above surface function to further describe the optical propertiesof the diffractive element.

FIGS. 5-8 shows simulated light distributions at the fiber and feedbackplanes for the transfer lens of the present invention and the prior arttransfer lens design for a multimode laser source (e.g., a multimodeVCSEL source).

FIG. 5 is a spot diagram for the transfer lens of the present inventionfor illustrating favorable launch conditioning. It is noted that thetransfer lens of the present invention provides very efficient couplingand also avoids the center of the fiber. Specifically, the diffractivesurface 150 provides favorable launch conditions so that light launchedinto the optical fiber avoids the index anomalies on the axis of theoptical fiber and at the core-cladding interface. FIG. 6 is a spotdiagram for the transfer lens of the present invention for illustratingreflection management. It is noted that the transfer lens of the presentinvention provides very good feedback management by directingreflections away from the light source.

FIG. 7 is a spot diagram for a prior art transfer lens that exhibitsincreased energy launched along the axis of the fiber. In contrast toFIG. 5, the prior art transfer lens provides poorer launch conditioningsince there is more energy launched into the center of the fiber opticcable. As described previously, it is disadvantageous to launch energyinto the center since the manufacturing defects therein may adverselyaffect or decrease the bandwidth of the system.

FIG. 8 is a spot diagram for a prior art transfer lens for illustratingpoor reflection management. In contrast to FIG. 6, the prior arttransfer lens provides poor reflection management since there is morelight reflected back into the light source. Consequently, the lightsource may suffer from stability problems due to poorly managedfeedback.

In the foregoing specification, the invention has been described withreference to specific embodiments thereof. It will, however, be evidentthat various modifications and changes may be made thereto withoutdeparting from the broader scope of the invention. The specification anddrawings are, accordingly, to be regarded in an illustrative rather thana restrictive sense.

For example, the diffractive element of the present invention has beendescribed in the context of launching light from a multi-mode laser intoa fiber optic medium. However, it is to be appreciated that thediffractive element of the present invention can be applied to manydifferent applications in a wide number of different areas. Thediffractive element of the present invention is beneficial to anyapplication where light needs to be coupled from a light source with acomplex light distribution to another light conducting medium. Forexample, the diffractive element of the present invention can beutilized to transfer light between two multi-mode fibers.

What is claimed is:
 1. An optical element for coupling a light source toa light conducting medium comprising: a diffractive surface that isdefined by a surface function that includes a first phase functionhaving angular symmetry and a first m value, and a second phase functionhaving radial symmetry, a cusp region and a second m value; wherein thecusp region has a discontinuous slope therein; wherein the first m valueand the second m value are selectively adjustable to control launchconditions and manage reflections.
 2. The optical element of claim 1wherein the first phase function is a spiral phase function; and whereinthe second phase function is a cone phase function.
 3. The opticalelement of claim 2 wherein the spiral phase function can be expressed asfollows: φ=m _(S)*θ where ‘m_(S)’ is a real number that describes howfast the phase changes as one traverses a circle about the center of theaperture; wherein ‘θ’ is an angular coordinate; and the cone phasefunction can be expressed as follows: φ=2πm _(C)*ρ where ‘m_(C)’ is areal number that describes how fast the phase changes as one traverses aradial line from the center of the aperture; wherein ‘ρ’ is a normalizedradial coordinate: wherein ρ is equal to 1 at the edge of the aperture,and ρ is equal to zero at the center of the aperture.
 4. The opticalelement of claim 3 wherein m_(S) is equal to =3 and m_(C) is equal to−2.
 5. The optical element of claim 3 wherein the values of m_(S) andm_(C) are selectively adjustable to control factors that include one ofcoupling efficiency, misalignment tolerances, and the amount offeedback.
 6. The optical element of claim 3 wherein the values of m_(S)and m_(C) are selectively adjustable to suit the requirements of aparticular optical application.
 7. The optical element of claim 2wherein the cone phase function includes a cross section that is one ofa generally concave profile, a generally triangular cross-section, agenerally convex profile, an inverted generally concave profile, aninverted generally triangular cross-section, and an inverted generallyconvex profile.
 8. The optical element of claim 1 wherein the opticalelement provides reflection management so that light reflected from theend of the optical fiber is not directed to a location at which light isemitted by the laser; and wherein the optical element provides favorablelaunch conditions so that light launched into the optical fiber avoidsindex anomalies along the axis of the optical fiber.
 9. The opticalelement of claim 1 further comprising: an optical surface for focusingthe light onto the optical fiber; and wherein the diffractive surfacereceives and collimates the light originating from a light source. 10.The optical element of claim 1 further comprising: packaging for housingthe light source; wherein the diffractive surface is disposed in thehousing.
 11. The optical element of claim 1 further comprising: a thirdphase function that includes one of a lens phase function, an aberrationcontrol phase function, a prism phase function, and a grating phasefunction.
 12. A method for manufacturing a diffractive surface for usein an optical element comprising: defining a first phase function havingangular symmetry and a first m value; defining a second phase functionhaving radial symmetry, a cusp region and a second m value; wherein thecusp region has a discontinuous slope therein; defining a surfacefunction that includes the first phase function and the second functionby selecting values for the first m value and the second m value tocontrol launch conditions and manage reflections; and employingsemiconductor processing techniques to manufacture a diffractive surfacefor use in the optical element in accordance with the surface function.13. The method of claim 12 further comprising: adding a third phasefunction to the surface function; wherein the third phase functionincludes one of a lens phase function, an aberration control phasefunction, a prism phase function, and a grating phase function.
 14. Themethod of claim 12 wherein the optical element couples light from alight source to a light conducting medium; and wherein defining asurface function that includes the first phase function and the secondfunction includes selectively adjusting the first m value and the secondm value to increase one of coupling efficiency between the light sourceand the light conducting medium and misalignment tolerances between thelight source and the light conducting medium.
 15. An optical element forcoupling a light source to a light conducting medium comprising: adiffractive surface that is defined by a surface function that includesa first phase function having angular symmetry, and a second phasefunction having radial symmetry and a cusp region; wherein the cuspregion has a discontinuous slope therein; and an optical surface forfocusing the light onto the light conducting medium; wherein thediffractive surface receives and collimates the light originating fromthe light source.
 16. The optical element of claim 15 wherein the firstphase function is a spiral phase function; and wherein the second phasefunction is a cone phase function.
 17. The optical element of claim 16wherein the spiral phase function can be expressed as follows: φ=m_(S)*θ where ‘m_(S)’ is a real number that describes how fast the phasechanges as one traverses a circle about the center of the aperture;wherein ‘θ’ is an angular coordinate; and the cone phase function can beexpressed as follows: φ=2πm _(C)*ρ where ‘m_(C)’ is a real number thedescribes how fast the phase changes as one traverses a radial line fromthe center of the aperture; wherein ‘ρ’ is a normalized radialcoordinate; wherein ρ is equal to 1 at the edge of the aperture, and ρis equal to zero at the center of the aperture.
 18. The optical elementof claim 17 wherein m_(S) is equal to =3 and m_(C) is equal to −2. 19.The optical element of claim 15 wherein the optical element providesreflection management so that light reflected from the end of the lightconducting medium is not directed to a location at which light isemitted by the light source.
 20. The optical element of claim 15 whereinthe optical element provides favorable launch conditions so that lightlaunched into the light conducting medium avoids index anomalies alongthe axis of the light conducting medium.